It is well known that excavation work for constructing a subway or a basement of a building is started by excavating holes along the borderline to a designed depth on the basis of technical drawings, and then vertical piles are installed into the excavated holes. After the installation of the vertical piles, excavation is partially carried out, and then main girders and cover plates are placed. After the placement of the cover plates, the additional excavation, and the placing of the wales and the supporting beams are alternately carried out. The aforementioned works are repeated in order to set up the support system in the excavated ground.
The aforementioned H-piles are usually used as the vertical piles in the support system. Alternatively, concrete may be filled into the excavated holes. Additionally, the steel piles and the concrete piles may be simultaneously used, or sheet piles may be used. And, preflex beams may be used as the vertical piles, and the H-piles may be attached to the sheet piles, to strengthen the sheet piles. However, the basic principle for supporting the excavated ground is same for the aforementioned works.
FIG. 1 is a cross-sectional view of an excavated space formed according to a conventional support system showing the arrangement of piles in the excavated space. As shown in FIG. 1, the temporary structure basically comprises: vertical piles 10 for supporting both excavated sidewalls; wales 50 for interconnecting the vertical piles 10; and struts 20 for supporting the wales 50 and the vertical piles 10. Additional intermediate piles may be placed between the vertical piles 10 when the width of the excavated space is relatively large. At the upper ends of the vertical piles 10, main girders 40 are placed, and on which cover plates 45 are securely placed so that vehicles can pass over the cover plates 45. Earth retaining plates are also inserted between the vertical piles 10 which prevent earth from collapsing between the vertical piles 10. A concrete structure 60 is built in the excavated space.
In the conventional support system, the pressure of earth and load applied to the struts 20 are repeatedly calculated to design the struts in such a manner that the struts withstand the maximum load applied to the beams when the temporary support structure is designed. As a result, a large number of struts 20 are required. In most cases, the struts 20 are closely arranged, for example, at intervals of approximately 2–3 m. The struts 20 closely arranged as described above are primary obstacles to delivering construction materials to the work place, bringing in heavy equipment and carrying out the support works. Also, the struts 20 extremely obstruct a molding work and a steel reinforcing work when the concrete structure 60 is built. For example, a plurality of holes is formed in the concrete structure 60 due to the struts 20, whereby the water-tightness and durability of the finished underground structure become severe problems.
FIG. 2 is a plan view showing a lengthy excavated space in one direction, which is supported by a conventional support system, and struts 20 arranged in the transverse direction. Generally, the struts 20 are closely arranged at intervals of approximately 2–3 m in the longitudinal direction. Between the struts are interposed, at intervals of approximately 20–30 m, bracing members 70 for preventing transverse displacement of the struts 20. As mentioned above, the struts 20 closely arranged are serious obstacles to delivering construction materials to the work place and carrying out the support works with heavy equipment.
There is an earth anchor system for supporting steel piles instead of the aforementioned struts, which is one of the support systems for constructing underground structures. According to this system, inclined holes are drilled into the ground behind the piles, tendons or high strength steel bars are inserted into the drilled holes, the ends of the inserted tendons or bars are anchored by means of a mechanical method or a chemical method, such as epoxy or cement grouting, and then the tendons or bars are tensioned and fixed to the steel piles. This system has an advantage in that the inner space of the temporary structure is very spacious, so that the earth works and the support works are easily carried out. On the other hand, this system has a disadvantage in that many of the tendons have to be placed in the neighbor's private properties when this system is applied in a crowded city, thus requires a formal consents from the neighbors. Also, the cost of construction is relatively high.
Korean Utility Model Registration No. 258949 discloses a method using truss for removing struts, which pass across the excavated space. This method is expected to be applied to the case where the depth of the excavated ground is relatively small. H-beams are doubly placed in a horizontal plane near the surface. The H-beams are reinforced with vertical beams and inclined beams so that the earth pressure is supported by truss placed at the upper part of the temporary structure. This method has been proposed to solve the difficulty in excavating and constructing the structure, which occurs due to the many support beams of the temporary structure for supporting the ground. Consequently, this method is useful for a relatively wide structure at the bottom and a relatively narrow structure at the top.
Korean Patent No. 188465 and Korean Utility Model Registration No. 247053 disclose a method for reinforcing wale by means of pre-stressing with straight tendon. In this method, an additional prestressed wale is placed on top of the existing wale so that the distance between the support beams can be increased. One method is using an additional wale, and the other method is to reinforce the existing H-beam's flange. It is expected that these two methods are effective to increase the distance between the support beams. However, because the tendons are straight, a constant support bending moment exists only in the middle part of the beam, which is different from the moment induced by the earth pressure. The different shape of the two moments restricts the length of the wale short.